How to calculate the current when a three-phase asynchronous motor operates with one phase missing?
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There is a 10KW three-phase AC motor whose load is a fan, and it operates in a delta connection under normal conditions. During normal operation of this motor (that is, in delta connection), if one phase of the power supply is suddenly disconnected, resulting in operation with a missing phase, what will be the current in the remaining two phases? (Assume: the current during normal triangular operation is 20A) Last edited by faner on 2009-3-7 23:25]
It’s hard to say; it depends on the load. It is certain that overloading is likely, unless it was originally under a fairly light load.
When a motor operates with one phase missing, the rotating magnetic field in the stator becomes severely unbalanced, which generates negative-sequence currents in the stator. The negative-sequence magnetic field induces a voltage of nearly 100 Hz in the rotor, causing the rotor current to increase sharply and leading to severe heating of the rotor. The motor’s load-carrying capacity drops dramatically under such conditions; it absorbs a large amount of active power from the power grid, resulting in a sharp increase in stator current. The severe unevenness of the magnetic field also causes significant vibration in the motor, which can damage the bearings and the frame. As a result, a motor operating with one phase missing under rated load will stop immediately due to stall, and even at light loads, the slip will be quite high. If the protection does not activate in time, the motor will be damaged; phase loss protection should generally be installed on motors. A rough estimate is that the current after a phase loss (the maximum value when stall occurs) is about 3-4 times the rated current. In practical applications, calculating the operating current under the phase-loss condition is not very meaningful. (Overheating causing insulation damage is the direct cause of motor burnout, whereas motor heating under a phase-loss condition is not caused solely by overcurrent.)